Of special interest as cations are the viologens, some of which, together with a large number of anions, are described and claimed in British Patent Specification No. 1,376,799 (U.S. Pat. No. 3,854,794) (German Pat. No. 2,260,048). In a paper by Schoot C.J., Ponjee J.J. et al, 1973 J. Appl. Phys. Vol. 23, p. 64, the particular electrolyte studied wad dihelpylviologen dibromide (N,N', di-hyptyl, 4,4' bipyridinium dibromide). On reduction of the viologen, a deep blue precipitate is formed with the bromide anion at the display electrode. At the anode a reverse reaction occurs. It has been found that the bromide anion corrodes and discolors the metal surface of the display electrode especially when silver, which is a highly preferred metal, is used, and that repeated reduction/oxidation cycles result in a non-erasible deposit on the cathode due to recrystallisation of the radical cation deposit.
In U.S. Pat. No. 3,950,077 to Raymond J. Jasinski, it is suggested that dihydrogen phosphate (H.sub.2 PO.sub.4.sup.-), or bicarbonate (HCO.sub.3.sup.-), be substituted for the bromide anion. The use of dihydrogen phosphate has solved the recrystallization and corrosion problems but the speed of the reactions at the electrodes is substantially reduced. In particular, erasure of the precipitate, besides being very slow, has been found to be highly temperature sensitive in the range of normal ambient temperatures, the erasure time increasing as temperature falls. Bicarbonate is not preferred by Jasinski.
It has been found that the precipitate formed with dihydrogen phosphate is highly resistive due, it is thought, to the unpolarizable nature of the dihydrogen phosphate molecule.
Jasinski, in his paper "The Electrochemistry of Some n-Hyptylviologen Salt Solutions" published in the Journal of the Electrochemical Society, May 1977, p. 637-640, considers besides H.sub.2 PO.sub.4.sup.- and HCO.sub.3.sup.-, several other anions including SO.sub.4.sup..dbd.,F.sup.-, formate, and acetate.
British Patent Specification No. 1,314,049 (U.S. Pat. No. 3,712,709) proposes as cations N(p-cyanophenol) substituted derivatives of a compound having two conjugated nitrogen containing aromatic rings, and as anions chloride and ferrocyanide.
In papers delivered at the Fall Meeting of the Electrochemical Society, Oct. 17-22, 1976, at Las Vegas, J. J. Ponjee and J. Bruinink propose the use of a new viologen cation - tetramethyl bis - 4(1-benzyl pyridin-4'-yl)pyridinium - with a large anion such as BF.sub.4.sup.- or CIO.sub.4.sup.-. These electrolytes seem to require a SnO.sub.2 electrode.